The short answer is that you can't. This image from MDN's explanation about stacking context explains it well:
There has been talk about escaping stacking context using position: fixed but it seems this is not happening just yet (see this fiddle and the question that generated it).
Alternate Solution:
For you, a possible alternative solution would be to nest Parent-1 inside Parent-2 and then use position: absolute to put Parent-1 wherever you want it.
